There are several known methods of forming gyroscopes, accelerometers, and other MEMS devices including structure suspended above a substrate.
According to commonly owned U.S. Pat. No. 5,492,596, incorporated herein by this reference, a silicon wafer is etched at desired locations to define the portions of the device which are suspended above a glass substrate. Boron is then diffused into the silicon wafer to a depth which will define the thickness of the device. The glass substrate is then prepared to include electrodes and the silicon wafer, diffused with boron, is bonded to the glass substrate. Finally, the silicon substrate is thinned by etching down to the boron diffusion layer.
The use of boron as an etch stop layer, however, may limit the thickness of the resulting device because boron diffusion is currently limited to a thickness of approximately 20 μm. Boron diffusion is limited in thickness by the long diffusion time and high diffusion temperatures required to obtain a deep diffusion profile with a boron concentration of greater than 5×1019 cm−3. The boron doping gradient generated by the diffusion process also creates a non-uniform stress gradient through the thickness of the micro-mechanical structure which has been shown to cause the structures to bend. Heavily boron doped silicon produced by diffusion is known to have a high density of dislocations caused by lattice mismatch of the boron doped layer with the undoped silicon substrate.
Deposition of thick boron doped epitaxial layers is also expensive. Heavily boron doped epitaxial layers have been demonstrated with a thickness of up to 50 μm but at a high cost (e.g., epitaxy cost can exceed $300.00 per wafer). Epitaxy costs are expected to increase as the thickness is increased. For thick epitaxial silicon deposition, the deposition process is often interrupted several times to perform chamber cleaning. Also, silicon epitaxy with boron exhibits high strains that can curl waters. If germanium is used to lower the stress, the result is a low mechanical quality factor which is disadvantageous in many applications such as vibrating angular rate sensors and resonators.
According to commonly owned U.S. Pat. No. 6,673,694, also incorporated herein by this reference, boron diffusion is not required if a silicon-on-insulator (SOI) type wafer is used.
Still, however, the basic structure of the device is preferably formed before wafer thinning. If features such as access ports are present, wet chemical etches cannot typically be used. Thus, although the method of the '694 patent has proven useful, there is still room for improvements and/or alterations.
In addition, the EDP etch required to dissolve the undoped portions of the silicon wafer is a highly corrosive wet chemical etch necessitating special exhaust and chemical handling procedures.
Finally, sticking is a major failure mode for devices fabricated by the dissolved wafer process. Because the sensors are released in a wet chemical environment, liquid surface tension has an opportunity to draw released silicon structures to the glass substrate. After EDP silicon release, a series of wet solvent rinses are required to remove etch residues which are sometimes suspected as a cause for stiction.